Days before the contentious U.S. Senate election in Indiana, outside groups have spent upward of $64.6 million on the race.

Since the May primary, outside groups, which are not affiliated with any campaign, have put money into the race between Democratic Sen. Joe Donnelly and Republican Mike Braun, an Evansville business owner and former state legislator, according to outside expenditure data from the Federal Elections Commission. The more than $64.6 million put toward the race is 700 times more than the amount spent during the 2010 Senate election.

“This is a phenomena that has been with us a while now,” said Edward Carmines, a professor of political science at Indiana University in Bloomington. Carmines is also the director of the Center on American Politics and research director of the Center of Representative Government at Indiana University.

Outside spending on elections has happened for decades, Carmines said, but accelerated in recent years in close elections. Large amounts of money have flowed into states like Indiana during close races for the Senate, U.S. House of Representatives or governor’s office, Carmines said.

“Outside money has really become a mark of high-profile Senate races,” Carmines said.

The top spenders to date in 2018 are: Senate Leadership Fund with $17.2 million; the Senate Majority PAC with $14.5 million; the DSCC with $6.8 million; Majority Forward with $5.2 million; the NRSC with $4.7 million; Priorities USA Action with $3.8 million; America First Action Inc. with $2.8 million; the National Association of Realtors PAC with $2.5 million; the Black Progressive Action Coalition with $2.1 million; and the National Rifle Association of America Political Victory Fund with $1.2 million.

In 2016, outside groups spent $45.9 million on the Senate race between Republican Todd Young and Democrat Evan Bayh.

In Indiana Senate races, the outside spending in the state has gone from $90,234.88 in 2010 to $25.5 million in 2012 and $45.9 million in 2014.

“Outside money has really become the rule now,” Carmines said.

The volume of outside spending has accelerated in the last 10 years, Carmines said.

The 2010 Citizens United case took the lid of outside spending, Carmines said, and it allowed money to come in from organizations with lax reporting requirements for the sources of those funds.

Carmines said the result of Citizens United was that it almost encouraged outside groups and individuals to put large amounts of money into races.

“Citizens United, in a way, opened the flood gates,” Carmines said. “It’s now become par for the course.”

The Senate Leadership Fund is so far the largest outside spender in the 2018 Senate race, according to FEC records.

The PAC has spent $17.2 million on online advertising; postage, printing, production; and TV or media placement. Roughly 83 percent of its spending has been in opposition to the re-election of Donnelly.

The Senate Leadership fund spent $11.9 million in 2016, according to the FEC.

The Senate Leadership Fund did not respond to a request for comment.

Historically, groups have put money to support particular candidates, according to Carmines, often bolstering candidates’ own spending and efforts.

Outside groups tend to put money toward both supporting candidates and attacking others, Carmines said. Outside groups want to support a candidate, Carmines said, but are motivated to take down opposition.

“It’s even more they want to defeat the opposition,” Carmines said.

Of all the money spent, 180 expenditures were to support Braun; 131 were opposed to Braun; 79 were in support of Donnelly; and 207 were in opposition to Donnelly.

The Senate Majority PAC spent its $14.5 million on media production and buys, according to the FEC data, roughly 84 percent of its money went toward opposing Braun.

“Throughout his time in the Senate, Joe Donnelly has been a tireless, independent fighter for Hoosiers; placing their needs ahead of party politics. Mike Braun, however, has amassed a business record built on putting his own profits before hardworking Hoosiers,” said Hannah Hurley, spokesperson for the Senate Majority PAC.

“This race comes down to what’s best for middle class Hoosiers and the only candidate who will go to bat for them is Joe Donnelly,” Hurley said.

The Senate Majority PAC spent $6.1 million in 2016, according to the FEC.

The National Association of Realtors PAC spent $2.5 million so far. All of its money has gone toward support Donnelly.

“This election cycle, NAR is supporting hundreds of candidates for federal office from both parties and in every corner of the country,” said Shannon McGahn, senior vice president of government affairs for the National Association of Realtors. “Our decisions to support candidates are based on incumbents’ voting records, answers to the Realtor PAC questionnaire, and positions that signify support for home ownership and real estate investment.”

“Our public support in the Indiana Senate race is no different, as we continue to work to ensure the issues that matter most to the real estate industry remain a priority for America’s lawmakers,” McGahn added.